Many industrial and scientific operations today depend upon filtration to separate valuable, nutrient or metabolic substances from waste or otherwise unusable products. These operations include direct osmosis, reverse osmosis and ultrafiltration methods; all involve passage of fluids over semipermeable membranes which allow the fluid to be separated from small dissolved molecules colloids, and suspended solids. Membrane-retained components are collectively called the concentrate or retentate. Materials permeating the membrane are called the filtrate or permeate.
Direct or natural osmosis utilizes a semipermeable membrane flanked on one side by a concentrated solute solution and on the other by a less concentrated solution. Osmotic forces will tend to equalize the solute concentration by passing water or other solvent through the membrane, while the solute cannot permeate the membrane. In reverse osmosis, application of pressure to the concentrated solution will force pure water or solvent back through the membrane, thereby concentrating the solute. In ultrafiltration, on the other hand, pressure is applied to a solution to force the solvent through the semipermeable membrane. That is, ultrafiltration is pressure-driven osmotic filtration on a molecular scale.
In addition to purely osmotic membranes which involve passage of solvent in one direction or the other, dialyzing membranes permit small solute molecules and ions to permeate the membranes. Dialysis basically is a membrane transport system in which solute molecules are exchanged between two liquids. For example, globular proteins in solution can easily be separated from low-molecular weight solutes utilizing a semipermeable membrane to retain protein molecules yet allow small solute molecules and water to pass through.
The largest contemporary use of dialysis is in hemodialysis, the treatment of the blood of persons with end-stage renal disease in which the kidneys are no longer capable of removing products of metabolism from the blood and excreting them. Hemodialyzers or "artificial kidneys" have now been used for almost 40 years to treat patient with severe renal failure. Many thousand persons with permanent renal failure or even total kidney removal are maintained in health for years at a time, their lives depending entirely on the artificial kidney.
In an artificial kidney, blood flows continually between membrane surfaces (the blood compartment); on the other side of the membrane is the dialyzing fluid (the dialysate compartment). The membrane is porous enough to allow all constituents of the plasma except plasma proteins to diffuse freely in both directions--from plasma into the dialyzing fluid and from the dialyzing fluid back into the plasma. If the concentration of a substance is greater in the plasma than in the dialyzing fluid, there will be a net transfer of the substance from the plasma into the dialyzing fluid. The amount of the substance that transfers depends on: (1) the difference between the concentrations on the two sides of the membrane; (2) molecular size, the smaller molecules diffusing more rapidly than larger ones; and (3) the length of time that the blood and fluid remain in contact with the membrane.
Types of dialyzers include: (1) coil, which incorporates a membrane in the form of a flattened tube wound around a central, rigid cylinder core, with a supporting mesh between adjacent portions of the membranes; (2) parallel plate, which incorporates a membrane in tubular or sheet form supported by plates in a sandwiched configuration; and (3) hollow-fiber, which incorporates the membrane in the form of very small fibers having a microscopic channel running through them. Most sheet and hollow-fiber membranes are cellulosic, although other materials have also been prepared and evaluated.
Probably the most important technical innovation in dialysis technology in support of hemodialysis is the development of hollow-fiber dialysis membranes and devices. One known dialyzer configuration is a capillary flow dialyzer, comprised of multiple hollow-fiber membranes contained within a housing. The hollow-fiber membranes are cylindrical capillaries having a diameter of less than 1 .mu.m, and whose walls function as the semipermeable membrane, permitting selective exchange of materials across the walls.
Since 1980, the reuse of hemodialyzers has risen dramatically. In 1986, the Association for the Advancement of Medical Instrumentation (AAMI) issued guidelines for the recommended practice in the reuse of hemodialyzers (Association for the Advancement of Medical Instrumentation, "Reuse of Hemodialyzers," Arlington, Va., 1986), stating that safe multiple use of hemodialyzers may actually improve the quality of care and/or access to dialysis. In large measure, the acceptability of reuse has also depended on the development and use of hollow-fiber hemodialyzers, which can be reprocessed to be equivalent in function, cleanliness, and sterility to a new hollow-fiber hemodialyzer.
Reprocessing of hemodialyzers after patient dialysis depends ultimately on cleaning and disinfecting of the dialyzer. Each piece of equipment used for reprocessing must be appropriately designed, constructed and validated to perform its intended task. Types of reprocessing system vary from sophisticated microprocessor-controlled systems to hand-operated valving systems. In reprocessing, both the blood compartment and the dialysate compartment are flushed with cleaning agents, such as hydrogen peroxide or sodium hypochlorite, and rinsed with water. The water is filtered through a nominal 5 .mu.m (micron) filter and should have a bacterial colony count of less than 200 /ml and/or a bacterial lippolysaccharide concentration of less than 1 ng/ml as measured by the Limulus amebocyte lysate assay. The dialyzer should also be free of any visible clotted blood.
The rinsed and cleaned dialyzer must also be treated by a process that prevents adverse effects due to microbial contamination. Typically, the blood compartment of the dialyzer is sterilized or subjected to high level disinfection. In practice, the dialysate compartment is also subjected to the same process because germicides pass through the membrane of the device. Typically, formaldehyde is used as the germicide, although other chemical germicides may be used which are shown not to damage the integrity of the dialyzer and must rinse out of the dialyzer to below known toxic levels with a rinse-out period established for the particular germicide. Care must also be taken not to mix reactive materials such as sodium hypochlorite and formaldehyde.
Hemodialyzers, reprocessed in conformance with the AAMI specific guidelines and performance tests, have an average use number, i.e., the number of times a particular hemodialyzer has been used in patient dialysis treatment may be about 8-15. Considering that hemodialyzers are used at least three times per week per patient, it would be highly desirable to improve the reprocessing system to increase the use number of a hemodialyzer.
Treatment of water to improve its properties for a variety of industrial and residential applications has been widely discussed in the scientific literature and in patents issued in the United States and other countries. The variety of devices for such treatment is so great that a comprehensive review thereof will not be undertaken here, it being generally known that such systems have been proposed based on technologies including static and dynamic magnetic treatment, treatment using electrostatic fields, ultrasound, externally-generated heating radiation (such as microwave), directly injected electromagnetic radiation, and, of course, a variety of chemical treatment techniques.
The scientific basis for the effects of various water treatment techniques has been widely debated and discussed, and opinions in the scientific community vary dramatically about the potential for such treatment techniques on an industrial or commercial scale. For example, in the Soviet Union, magnetic treatment of water to assist in removal or prevention of scale has been reported. Favorable analysis of such treatment has been criticized by literature generated in the United States. Some of the theories discussed include one which advocates the hypothesis that magnetic treatment decreases the surface tension of the water molecules, thereby making the treated water "wetter" than untreated water. Another advocates the belief that the magnetic fields generated within the water act only on the impurities contained within the water. Others related to ionic charge theory, minor changes in pH, changes in the zeta potential or the like.
Despite such debate over the scientific basis of the treatment effect, a number of individuals and companies are continuing to suggest new types of treatment devices for previously discussed applications and new technologies for unrelated and surprisingly diverse applications.
Examples of magnetic water treatment devices include the following:
Stickler et al., U.S. Pat. No. 4,746,425 issued May 24, 1988 for "Cooling System for Magnetic Water Treating Device" and Stickler, et al., U.S. Pat. No. 4,659,479 issued Apr. 21, 1987 for "Electromagnetic Water Treating Device", both use a pipe core of alternating magnetic and non-magnetic sections with an electromagnet surrounding the pipe through which the fluid to be treated passes.
Another treatment system is disclosed in Larson, et al., U.S. Pat. No. 4,865,747, issued Sep. 12, 1989, for "Electromagnetic Fluid Treating Device and Method". An electromagnetic field having a voltage which operates in the range of 1 kHz to 1000 MHz is applied to a non-ferromagnetic conduit in which a ferromagnetic core is mounted. The core acts as a sacrificial anode and as a receiving antenna for the radiofrequency electromagnetic radiation.
A permanent magnet system is described in Mitchell, U.S. Pat. No. 4,808,306 issued Feb. 38, 1989 for "Apparatus for Magnetically Treating Fluids". A field generator is mounted on one side of a pipe, through which fluid to be treated passes, in a non-ferromagnetic housing. A magnet is embedded in the housing and has an odd number of sections of alternating polarity. For fuel treatment, the uppermost section is desirably a south polar magnetic field. If water is to be treated, a north ferromagnetic plate mounted adjacent to but outwardly from the pipe is used for increasing magnetic field strength. Mitchell indicates that his device can lead to fuel consumption savings, to maintain minerals and other contaminates of water in solution (softening of water), prevention of scale and rust and to improve the taste, cleaning and solvent properties of water.
Additional patents which refer to the use of magnets to treat water include Carpenter, U.S. Pat. No. 4,367,143 issued Jan. 4, 1983, for "Apparatus for Magnetically Treating Liquid Flowing Through a Pipe and Clamping Means Therefor". This patent discusses applying a plurality of strips of ferromagnetic material contained in a shoe member on the outside of a pipe, the number of magnetic strips and the power of the magnets being selected for a particular pipe size. The polarities of the magnets in each strip are aligned in the same way, e.g., all south polar ends being oriented upstream with respect to water flow. See, also, Kulish, U.S. Pat. No. 4,605,498 issued Aug. 12, 1986 for "Apparatus for Magnetic Treatment of Liquids". The magnet arrangement of this patent (surrounding arcuate shape magnets) is such that the south pole magnetic fields are concentrated toward the axis of a pipe through which liquids to be treated pass, while the north poles are directed radially outwardly.
A unique magnet arrangement for water treatment is disclosed in U.S. Pat. No. 4,888,113 issued to Holcomb on Dec. 19, 1989 for "Magnetic Water Treatment Device". In this patent, Holcomb discusses the use of a plurality of rectangular magnets attached to the exterior of a pipe. The magnets are arranged in pairs adjacent the pipe such that the positive pole of one pair is oriented to one end of a support housing and the negative pole is oriented toward the other end of the housing. Another similarly constructed housing is secured to the opposite side of the pipe, the second housing also from those in the first housing. Thus the positive pole of the first set faces the negative pole of the second set to cause an "attractive" mode of magnetic flux treatment. Applications such as scale prevention, as well as use in washing machines, swimming pools, ice rinks, livestock watering, and coffee brewing are suggested. The patent also suggests that the taste of treated water is superior to that of untreated water. The patent further mentions that the magnetic force fields can be generated through wound iron coils coupled to a DC generator.
Another water treatment technique is that disclosed in U.S. Pat. No. 4,865,748 issued Sep. 12, 1989 to D. Morse and entitled "Method and System for Variable Frequency Electromagnetic Water Treatment". In this device, a non-insulated conductor in direct contact with a fluid to be treated is coupled to a generator of electromagnetic radiation, preferably in the radio frequency range. According to the patent, the radiation is injected at a frequency which is related to the electromagnetic radiation absorption or emission profile of the particular system being treated. This patent also focuses on the use of that system for the elimination and prevention of scale build-up in boiler systems and the like.
These patents are representative of the wide diversity of treatment techniques discussed in the art and it is important for a more complete understanding of the prior art to read the "Background" sections of each of the foregoing patents. Also the tabular listings of art cited against such patents should be reviewed. Each of the background disclosures and listings is incorporated herein and is made available by the copies of the patents supplied herewith.